In the field of measurement of bio-magnetism for diagnosing a patient on the basis of a feeble magnetism generated from a human body, and in the field of a magnetic prospecting for exploration of mineral resources on the basis of a feeble magnetism generated from rocks, for example, an external magnetism such as geomagnetism present in the environment of an object of magnetic detection and in the environment of a magnetic detector, and a leakage magnetism generated from various electrical appliances, forms a factor causing decrease in a detecting accuracy of such a feeble magnetism.
In order to improve the detecting accuracy of a feeble magnetism, therefore, it is necessary to magnetically shield the object of magnetic detection and the magnetic detector from the external environment by means of a magnetic shield so as to prevent the external magnetism from coming into the object of magnetic detection and the magnetic detector.
A magnetic shield is usually made of a ferromagnetic material such as iron or nickel. By covering the object of magnetic detection and the magnetic detector by means of the magnetic shield made of a ferromagnetic material, most of the external magnetism passes along the magnetic shield without coming into the space surrounded by the magnetic shield, in which the object of magnetic detection and the magnetic detector are placed. A feeble magnetism is therefore detected at a high accuracy without being affected by the external magnetism.
However, while the magnetic shield made of the ferromagnetic material is effective in reducing a relatively strong external magnetism to an intensity of the order of that of geomagnetism, the magnetic shield of this type gives only a poor magnetic shielding effect when it is necessary to largely reduce the intensity of the external magnetism as compared with that of geomagnetism, as in the case of measurement of a feeble magnetism such as bio-magnetism, because the magnetic shield made of the ferromagnetic material has a residual magnetism.
With a view to overcoming this inconvenience, "Japanese Journal of Applied Physics" Vol. 28, No. 5, issued in May 1989, pp. L813-L815 discloses a magnetic shield made of a superconducting material, as a magnetic shield capable of giving an excellent magnetic shielding effect (hereinafter referred to as the "prior art").
A magnetic shield of the above-mentioned prior art is manufactured by compression-forming a powder of a composite oxide superconducting material such as YBa.sub.2 Cu.sub.3 O.sub.x to prepare a cylindrical green compact, and firing the thus prepared green compact.
According to the above-mentioned prior art, it is possible to obtain a high magnetic shielding effect under the Meissner effect, i.e., the diamagnetic effect of completely excluding the external magnetism, of the composite oxide superconducting material.
However, the above-mentioned prior art has the following problem: When applying the magnetic shield of the prior art for measurement of bio-magnetism, for example, it is necessary for the magnetic shield to have a large size sufficient to receive a human body. However, when it is attempted to manufacture a large magnetic shield from the composite oxide superconducting material, cracks tend to occur in the large green compact made of the composite oxide superconducting material when firing same. It is therefore very difficult to manufacture a large magnetic shield capable of receiving a human body by the application of the above-mentioned prior art.